Photonic crystals are generally understood to be three-dimensional dielectric structures which are opaque to electromagnetic radiation within a specific wavelength range, regardless of the direction of incidence. In this context, the wavelength range is substantially determined by the arrangement, form, and size of the structures. One type of photonic crystals is constituted of an evenly spaced, for example, matrix-type, array of free-standing dielectric micro-columns or micro-cylinders having very small diameters and a substantial height relative thereto. For visible and infrared light, typical micro-cylinder diameters are in the range of one hundred to a few hundred nanometers.
To fabricate photonic crystals having micro-cylinders, a substrate is needed to support the micro-cylinders. For this, in available methods, inter alia, fused quartz or silica is mostly used due to the mechanical and chemical stability it offers. In the following, it is understood that “photonic crystals” refer to the three-dimensional structure having micro-columns or micro-cylinders, along with the substrate that supports them.
Because of their unusual properties, photonic crystals may be suited for manufacturing optical components, such as very narrow-band filters, modulable filters, add-drop filters, and integrated optical structures having 90°-deflection. Components of this kind are used in DWDM technology (dense wavelength division multiplexing) for transmitting data in optical waveguides, where electromagnetic radiation of various wavelengths is used as a carrier. At the present time, the channel separation or spacing between simultaneously used, adjacent channels is 0.4 nm, wavelengths in the range of about 1550 nm being used. In the future, to achieve a higher bandwidth for transmissions via optical waveguides, the channel separation may drop to 0.2 nm or less. This requires components whose filter properties are extremely stable, particularly in response to substantial temperature fluctuations.
Under substantial temperature fluctuations, the optical properties of photonic crystals and, thus, in particular, also the transmission wavelength of filters based on photonic crystals can markedly change.